Foundry alloy

ABSTRACT

An aluminium-based alloy having 6.5-7.5 wt. % silicon and 0.35-0.50 wt. % magnesium as the major alloying elements and a method of manufacturing an article from the alloy are disclosed. The alloy is characterised by a microstructure in which β phase (Al 5 SiFe) that forms during heat treatment as a transformation product of π phase (Al 8 Si 6 Mg 3 Fe) is the sole or predominant iron-containing phase in the alloy.

[0001] The present invention relates to an improved foundry alloy and toa method of producing an improved foundry alloy. In particular, theimproved foundry alloy is an aluminium-based alloy,

[0002] Primary metal based foundry alloys are largely used forstructural or safety type applications where there is a requirement forhigh and consistent mechanical properties. The majority of componentsmade from aluminium foundry alloys are made from hypoeutecticaluminium-silicon-magnesium alloys containing a nominal silicon level of7% by weight (601 and 603 designations). In simple terms these alloysare a composite of hard, discontinuous silicon particles and large,brittle iron intermetallics embedded in a ductile aluminium matrix.

[0003] There are three registered Australian compositions forstrontium-modified aluminium—7% silicon alloys. These are given inTable 1. The magnesium content of the alloys covers the range 0.25 to0.4 wt % (601 alloys) and 0.45 to 0.7 wt % (603 alloys). The addition ofmagnesium allows castings to be heat treated to form magnesium silicideprecipitates. These harden the matrix of the alloy to obtain the desiredcombination of strength and ductility. TABLE 1 Registered alloycomposition for strontium modified 601/603 type foundry alloys AlloyOther Other Code Si Fe Cu Mn Mg Zn Ti Each Total Al AC601 6.5-7.5 0.200.05 0.05 0.30-0.40 0.05 0.20 0.05 0.15 Rem CC601 6.5-7.5 0.20 0.05 0.050.25-0.35 0.05 0.20 0.05 0.15 Rem AC603 6.5-7.5 0.15 0.05 0.03 0.45-0.7 0.05 0.20 0.05 0.15 Rem

[0004] The main impurity found in these alloys is iron. The ironsolidifies from the eutectic liquid into a number of brittle phases.

[0005] The two major iron-containing phases found in these alloys arethe π phase (Al₈Si₆Mg₃Fe) which is the predominant phase formed in highMg content alloys and the β phase (Al₅SiFe) which forms in low magnesiumcontent alloys. The π phase forms into a script morphology while the βphase is less voluminous and forms into acicular plates. Both phases aredetrimental to mechanical properties. High Mg contents (ie greater than0.6 wt % Mg) are desirable to provide higher strength, but the presenceof π phase at high Mg contents causes the ductility of the alloys tounfavourably decrease.

[0006] Conventional theories on the micro-mechanics of failure ofpremodified 601 and 603 alloys state that the iron rich intermetallicphases are critical in determining the fracture toughness as the siliconparticles are small and round. Increases in the magnesium content ofthese alloys increase the amount of the π phase, which has a negativeimpact on the ductile properties of the alloys.

[0007] Further, as some magnesium is contained in the π phase, themaximum volume fraction of magnesium silicide precipitates cannot beobtained. Thus, the alloys do not achieve the maximum possible strengthconsistent with their magnesium content.

[0008] Also, as the magnesium content of an alloy increases themagnesium content of the π phase may change leading to even greatervolume fractions of the phase for a given Fe content.

[0009] It is thus concluded that the overall quality of an alloy, asgiven by the quality index, decreases as increasing volume fractions ofthe π phase forms at increased magnesium contents. The quality index isgiven by the formula:

Q.I.=UTS+150 log₁₀ E

[0010] where:

[0011] Q.I.=Quality Index (Mpa)

[0012] UTS=Ultimate Tensile Strength (Mpa)

[0013] E=Elongation at Fracture (%)

[0014] Attempts have been made to eliminate the π phase and thus removeits detrimental impact on mechanical properties.

[0015] By way of example, beryllium additions can be used to precipitatethe iron impurity as part of the BeSiFe₂Al₈ phase. Thisberyllium-containing phase forms in preference to the π phase, leadingto alloys with improved mechanical properties. Unfortunately, there areserious health hazards associated with using beryllium. Consequently,beryllium modification is not widely practised and the deleteriouseffect of the π phase on alloy quality remains.

[0016] Other attempts to modify the iron-containing phases, for exampleby using Mn, have been tried in secondary alloys but have not been usedin primary alloys.

[0017] It is an object of the present invention to provide an improvedfoundry alloy.

[0018] In accordance with the present invention this object is achievedby an alloy having a microstructure in which β phase that forms duringheat treatment as a transformation product of π phase is the sole orpredominant iron-containing phase. The reduction in π phase results inan improvement in ductility. Moreover, the β phase that forms as thetransformation product has a fine structure that improves ductility.Further, the reduction in π phase means that there are higher levels ofMg in solution which are available for precipitation during ageing toimprove the strength of the alloy.

[0019] In a first aspect, the present invention provides an alloy whichcomprises:

[0020] Si: 6.5-7.5 wt %

[0021] Fe: up to 0.20 wt %

[0022] Cu: up to 0.05 wt %

[0023] Mn: up to 0.05 wt %

[0024] Mg: 0.35 to 0.50 wt %

[0025] Zn: up to 0.05 wt %

[0026] Ti: up to 0.20 wt %

[0027] Balance: Al and other components, the other components comprise atotal of not more than 0.15 wt % and any single component of the othercomponents does not exceed 0.05 wt %, the alloy having a microstructurewhich includes a primary aluminium-containing matrix and one or moreiron-containing phases dispersed in the matrix, and wherein the sole orpredominant iron-containing phase is β phase that formed as atransformation product of π phase.

[0028] It is preferred that the dendrite arm spacing of the matrix be10-45 μm.

[0029] Where there is more than one iron-containing phase, preferablythe iron-containing phases also include π phase.

[0030] Preferably, the iron-containing phases include π phase in anamount up to 30 vol % of the iron-containing phases. The amount of πphase may be higher if the Mg content is in the upper end of the range.

[0031] The Mg content of the alloy is preferably 0.40-0.45 wt %. Withinthis Mg range, the alloy is a variant of the 601/603 type foundry alloy.It has been realised by the applicant that close control of themagnesium content to be between 0.40 and 0.45 wt % can lead to anincrease in alloy quality and improved mechanical properties. Inparticular, when the magnesium content is controlled to be between 0.40and 0.45 wt % the variation in alloy quality for a small change inmagnesium level is minimal. Thus, the consistency in the mechanicalproperties of the alloy is maximised.

[0032] The present invention also provides a method for manufacturing analloy article.

[0033] In a second aspect, the present invention provides a method formanufacturing an alloy article which comprises:

[0034] (a) providing a melt having a composition of:

[0035] Si: 6.5-7.5 wt %

[0036] Fe: up to 0.20 wt %

[0037] Cu: up to 0.05 wt %

[0038] Mn: up to 0.05 wt %

[0039] Mg: 0.35 to 0.50 wt %

[0040] Zn: up to 0.05 wt %

[0041] Ti: up to 0.20 wt %

[0042] Balance: Al and other components, the other components comprisinga total of not more than 0.15 wt % and any single component of the othercomponents not exceeding 0.05 wt %,

[0043] (b) casting said melt and solidifying a casting at a cooling ratethat produces a microstructure of an aluminium-containing matrix and πand β iron-containing phases dispersed in the matrix;

[0044] (c) solution heat treating the casting to at least partiallytransform π phase to β phase; and

[0045] (d) quenching the casting to form the alloy article.

[0046] It is preferred that the cooling rate be sufficient to produce adendrite arm spacing in the matrix in the casting of 10-45 μm.

[0047] Preferably, the sole or predominant iron-containing phase in thealloy article is β phase.

[0048] Where there is more than one iron-containing phase in the alloyarticle, preferably the iron-containing phases also include π phase.More preferably, the iron-containing phases in the alloy article includeπ phase in an amount of up to 30 vol % of the iron-containing phases.Higher levels of π phase may be present if the Mg content is at theupper end of the above range.

[0049] It is preferred that the step of solidifying the casting producesiron-containing phases that include a substantial proportion of the πphase and the subsequent solution heat treatment step is effective toconvert at least some and preferably a majority of the π phase to βphase to give a microstructure in the alloy article that includesiron-containing phases which are predominantly β phase.

[0050] The melt prior to casting may be at a temperature above theliquidus temperature of the alloy, with the melt having sufficientsuperheat to fill the mould, that is at a temperature of 680-720° C.

[0051] The solution treatment of the casting may be carried out at anysuitable temperature and for any suitable time to achieve a desiredlevel of transformation of π phase β phase. In any given situation, theselection of the parameters of temperature and tine will depend onvariables, such as the concentrations of magnesium and other elements inthe casting. By way of example, the applicant has found that forcastings having a Mg concentration of 0.5 wt %, solution treatment at540° C. for 2 or more hours produced desired levels of transformation ofπ to β phase.

[0052] After the solution heat treatment, the casting is preferablyquenched, more preferably quenched in hot water, such as hot waterhaving a temperature of 70-80° C.

[0053] After quenching, the alloy article is cooled to room temperatureand optionally subjected to an ageing heat treatment.

[0054] The ageing heat treatment may include heating the alloy articleto a temperature of 140-170° C. and holding at that temperature for 1-10hours. After the ageing heat treatment, the alloy article may be aircooled to room temperature.

[0055] Results to support the present invention are given in FIG. 1, inwhich plots of typical response surfaces derived from experimentallydetermined quality index data are shown. The three surfaces correspondto alloys that were cast at different solidification rates andthereafter solution treated and aged. Solidification rate is commonlymeasured by the as-cast dendrite cell size or secondary dendrite armspacing (DAS) but other methods exist. The results here use secondarydendrite arm spacing to indicate solidification rate, with a smalldendrite arm spacing corresponding to a high solidification rate.

[0056] It can be seen from FIG. 1 that:

[0057] (i) at the high solidification rate (=20 μm DAS) the alloyquality peaks at a magnesium level of 0.45-0.50 weight percent;

[0058] (ii) at the intermediate solidification rate (=40 μm DAS) thequality peaks at a magnesium level of 0.35-0.40 weight percent; and

[0059] (iii) at the low solidification rate (=60 μm DAS) the qualitymaximum occurs at a magnesium level of 0.25-0.30 weight percent.

[0060] Further, it can be seen from FIG. 1 that the magnesium level forthe peak quality is independent of the iron level for the iron levelsexamined. Also, the rate of change of the response surfaces withmagnesium is least near the peak in quality index. This means that thealloys at the peak are less sensitive to changes in magnesium than otheralloys. The peak quality from FIG. 1 corresponds well withmicrostructural evidence for small amounts of π phase in the alloy. Byincreasing the magnesium content of the alloy, it can be seen that insome circumstances improved quality results.

[0061] It should be noted that the present invention works best withthose casting designs or casting methods which produce highsolidification rates (≦45 μm DAS), such as permanent mould, mould chillmethods with sand, and squeeze casting. Indeed, the trend in theautomotive industry is to move away from thick section, lowsolidification rate (high DAS) castings towards lightweight castingswith thinner sections and higher solidification rates (low DAS).

[0062] The common belief prior to the present invention was that lowmagnesium levels produce high quality castings. The results shown hereconfirm this to be true at low solidification rates (FIG. 1c). However,at higher solidification rates, the magnesium contents covered by thisinvention show, surprisingly, improved alloy quality and thereforeimproved mechanical properties.

[0063] FIGS. 2(a) to 2(c) are photomicrographs of hypoeutectic alloyshaving a Si concentration of 7 wt % and various Mg concentrations whichwere cast at the same solidification rate (60 μm DAS), solution treated,and aged. FIG. 2 (d) is a photomicrograph of the as-cast alloy of FIG.2(c), ie before heat treatment.

[0064] In FIG. 2(a), the Mg content of the alloy is higher than the Mgcontent of the alloy of the present invention. The main phases shown inFIG. 2(a) are spheroidal silicon-containing phase and theiron-containing π phase.

[0065]FIG. 2(b) shows the microstructure of an alloy containing less Mgthan the alloy of the present invention. The phases present includespheroidal silicon-containing phase and iron-containing β phase. The βphase is present as structures of high aspect ratio dispersed throughoutthe matrix.

[0066]FIG. 2(c) shows the microstructure of an alloy of the presentinvention. The phases include spheroidal silicon-containing phases, asmall amount of π phase an β phase. The β phase is present as structuresof high aspect ratio clumped together. This is consistent with the βphase being formed by transformation of π phase during heat treatment.

[0067]FIG. 2(d) shows that prior to heat treatment the as-cast alloy ofFIG. 2(c) had regions of π phase. As is evident from FIG. 2(c) these πphase regions were largely transformed to β phase during heat treatment.

[0068] The drive for alloys with improved mechanical properties stemsfrom the major restraint that mechanical properties place on the designof the casting, or even if a cast alloy can be used to manufacture acertain component. The thickness of critical sections needs to besufficiently large that the cast component can operate without failure.Mechanical properties of the alloys therefore limit the minimum weightof a cast component. Further, the thickness of sections of a castingwill determine the time required for the casting to solidify. Forcertain casting methods, such as low pressure die casting, theproduction rate is often determined by the solidification rate as thecasting machine is tied up until the casting has fully solidified.Finally, the solution treatment, quench rate and ageing treatment of acast component may be tailored to its design so as not to induceunnecessarily high residual stresses. High residual stresses can causedistortion of the component requiring additional machining. Themechanical properties of the base alloy therefore affect all stages ofmanufacturing from design, to casting the component, heat treatment,machining, final weight and production rate.

[0069] The present invention therefore has the following more specificapplications:

[0070] (i) New markets for aluminium—7% silicon foundry alloys. Castalloys generally have inferior mechanical properties but lowermanufacturing costs compared to similar components made from wroughtalloys. The high mechanical property requirements of some componentsnecessitates the use of wrought alloys. The achievement of alloys of thepresent invention which have higher and more consistent mechanicalproperties than conventional alloys may allow the use of the alloy ofthe present invention to replace wrought alloys, or other cast alloys,for some components.

[0071] (ii) Cast components with thinner sections and lower totalweight. The improved and more consistent mechanical properties of thealloy of the present invention allows components with thinner sectionsto be designed and cast. Despite their thinner sections, thesecomponents can still operate without failure and will have a lower totalweight.

[0072] (iii) Cast components with an improved production rate. Castingswith thinner sections may require less time to solidify. Productionequipment will then be tied up for less time waiting for a component tosolidify. The production rate is thus increased.

[0073] (iv) Cast components with refined iron and silicon intermetallicphases. The solidification time of a casting strongly determines thecoarseness of the microstructure. Components with thinner sections andtherefore higher solidification rates (and lower solidification times)will have a more refined microstructure. This refining of themicrostructure will provide additional improvements to the mechanicalproperties of a casting, independent of the use of a superior alloy.

[0074] (v) Cast components with reduced heat treatment time. Castingswith thinner sections require less time to homogenise. Further, the timerequired for the casting to reach the solution treatment temperature orageing temperature will be less. This also benefits the production rateof components.

[0075] (vi) Cast components with increased bench rate. Thinner castingsmay quench more rapidly. This may lead to improved mechanical propertiesas it suppresses the formation of magnesium-silicide precipitates duringcooling. These improved properties are independent of any refinement ofthe microstructure or the use of a superior alloy.

[0076] It will be appreciated that the invention described herein issusceptible to variation and modifications other than those specificallydescribed. It is to be understood that the invention encompasses allsuch variations and modifications that fall within its spirit and scope.

1. An ally which comprises: Si: 6.5-7.5 wt % Fe: up to 0.20 wt % Cu: upto 0.05 wt % Mn: up to 0.05 wt % Mg: 0.3 to 0.50 wt % Zn: up to 0.05 wt% T1: up to 0.20 wt % Balance: Al and other components, the othercomponents comprise a total of not more than 0.15 wt % and any singlecomponent of the other components does not exceed 0.05 wt %, the alloyhaving a microstructure which includes a primary aluminium-containingmatrix and one or more iron-containing phases dispersed in the matrix,and wherein the sole or predominant iron-containing phase is β phasethat formed as a transformation product of π phase.
 2. The alloy definedin claim 1, wherein when the alloy includes more than oneiron-containing phase, the iron-containing phases also include π phase.3. The alloy defined in claim 2, wherein the π phase is up to 30 vol %of the iron-containing phases.
 4. The alloy defined in any one of thepreceding claims, wherein the Mg content of the alloy is 0.40-0.45 wt %.5. A method for manufacturing an alloy article which comprises: (a)providing a malt heaving a composition of: Si: 6.5-7.5 wt % Fe: up to0.20 wt % Cu: up to 0.05 wt % Mn: up to 0.05 wt % Mg: 0.35 to 0.50 wt %Zn: up to 0.05 wt % Ti: up to 0.20 wt % Balance: Al and othercomponents, the other components comprising a total of not more than0.15 wt % and any single component of the other components not exceeding0.05 wt %, (b) casting said melt and solidifying a casting at a coolingrate that produces a microstructure of an aluminium-containing matrixand π and β iron-containing phases dispersed in the matrix; (c) solutionheat treating the casting to at least partially transform π phase to βphase; and (d) quenching the casting to form the alloy article.
 6. Themethod defined in claim 5, wherein the cooling rate is sufficient toproduce a dendrite arm spacing in the matrix of between 10 and 4.5 μm.7. The method defined in claim 5 or claim 6, wherein the sole orpredominant iron-containing phase in the alloy article is β phase. 8.The method defined in claim 5, wherein when the alloy includes more thanone iron-containing phase in the alloy article, the iron-containingphases also include π phase.
 9. The method defined in claim 8, whereinthe π phase is up to 30 vol % of the iron-containing phases.
 10. Themethod defined in claim 5 or claim 6, wherein the step of solidifyingthe casting produces iron-containing phases that include a substantialproportion of π phase and the subsequent solution heat treatment step iseffective to convert a majority of the π phase to β phase to give amicrostructure in the alloy article that includes iron-containing phaseswhich are predominantly β phase.
 11. The method defined in any one ofclaims 5 to 10, wherein prior to casting the melt is at a temperatureabove the liquidus temperature of the alloy.
 12. The method defined inany one of claims 5 to 12, wherein the quenching step is in hot waterhaving a temperature of 70-80° C.
 13. The method defined in any one ofclaims 5 to 12, further includes an ageing heat treatment of the alloyarticle.
 14. The method defined in claim 13, wherein the ageing heattreatment includes heating the alloy article to a temperature of140-170° C., holding the alloy article at that temperature for 1-10hours, and air cooling the alloy article to room temperature.